In an internal combustion engine, the function of a butterfly valve is to regulate the flow of fresh air supplied to the cylinders; normally, a butterfly valve has a valve body housing a valve seat engaged by a butterfly body which is keyed on a shaft in order to rotate between an open and a closed position of the valve seat under the action of an electric actuator coupled to this shaft by means of a gear transmission. An elastic body (typically formed by a double spring) is also coupled to the shaft and exerts a torque on the shaft which tends to rotate the butterfly body towards the open position and which, in the absence of action by the electric actuator, causes the butterfly body to be disposed in a partially open position (commonly known as the limp-home position) as a result of the presence of an abutment surface which forms an abutment for the elastic body against which the opening movement caused by this elastic body is stopped.
Currently, the abutment surface is formed by a support body which is obtained by casting on the crude valve body; however, the sum of the tolerances in respect of the casting work, the joint molding of the shaft, the diameter of the butterfly body and the diameter of the valve seat cause a total air flow dispersion in the limp-home position of approximately ±18-20%. In some applications, this total air flow dispersion value in the limp-home position is too high; it has therefore been proposed to carry out precision machining of the support body, which precision machining makes it possible to reduce the total air flow dispersion in the limp-home position to approximately ±10-12%.
However, this precision machining is particularly costly and in any case does not make it possible to obtain a total air flow dispersion value in the limp-home position of less than ±10%. Moreover, in order significantly to vary the value of the air flow in the limp-home position (typically to be able to adapt the butterfly valve to different types of engine) it is necessary to modify the casting mould to vary the position of the support body; in general, a specific valve body and therefore a specific mould is required for each flow value, which obviously increases production costs.
In order to try further to reduce the total air flow dispersion value in the limp-home position, it has been proposed to replace the support body with a screw which is screwed through the valve body and has a head disposed outside this valve body and a free end forming the abutment surface against which the elastic body comes to abut. During the production stage, each butterfly valve is disposed on a test bench, where the value of the air flow in the limp-home position is measured in real time; in these circumstances, the axial position of the screw is adjusted by screwing or unscrewing the screw with respect to the valve body until the desired value of the air flow in the limp-home position is accurately obtained. Preferably, once the axial position of the screw has been adjusted, the screw is locked with respect to the valve body to prevent any subsequent displacement (typically as a result of the vibrations generated by the operation of the engine).
The use of a through screw does not make it possible, however, significantly to vary the air flow value in the limp-home position without modifying the casting mould.
FR2781525 discloses a motorized throttle butterfly with limp-home facility for use in motor vehicles and having a spring with two torsion zones on either side of bolt to set throttle in limp-home position; the first zone has its end connected to the throttle housing, and the second zone has its end coupled to a support fixed to the butterfly spindle. The two spring zones are on either side of a bolt that when engaged sets the throttle in the limp-home position.
EP1148225 discloses a throttle return mechanism for an electronically controlled throttle that provides for the precise setting of a limp home throttle blade position. The throttle return mechanism includes a return spring with two legs attached to a fixed shaft, and a bracket, which is attached to a drive mechanism and includes stops that engage the return spring as the bracket rotates about the fixed shaft; each stop is cam shaped and rotatable to provide for adjustment of the limp home throttle blade setting. When the drive mechanism is disabled the legs of the return spring will engage the stops on the bracket and rotate the throttle blade to the limp home position; the second leg of the return spring will rotate and hold the throttle valve in a limp home throttle position to allow a driver to maneuver the motor vehicle.